In the face of shrinking world reserves of high-grade iron ore, low-grade ores are used, which must be concentrated before further processing. The concentrated iron ore fines obtained cannot be used as such, for example in a blast furnace to produce pig iron or to produce sponge iron in a direct reduction iron plant (DRI).
The fines have first to be mixed with additives such as bentonite or other binders such as described for example in U.S. Pat. No. 5,685,893 issued to Field et al., and then shaped into small lumps, the pellets, which are then hardened by firing in a furnace at temperature up to 1350° C.
The total world production of such iron pellets is about 400 million metric tonnes per year, which corresponds to about 20% of the total iron ore production.
The most commonly employed method of heat-hardening, such as described for example in U.S. Pat. No. 3,947,001 issued to Leighton, is of disposing the iron ore pellets on a travelling grate to carry the pellets through a tunnel-like enclosure where they are progressively subjected to drying, preheating, firing, after-firing and cooling. An alternative technology, as described for example in U.S. Pat. No. 4,168,951 issued to Drugge, or as proposed by Metso, is to use a rotary kiln for the firing zone of the process instead of a moving grate.
The initial step of drying is carried out at moderate temperature to allow a gradual release of the moisture in the pellets. After drying, the pellets are fired at the required temperature to cause the pellets to fuse.
The process heat is provided by fuel burners, such as proposed by companies such as Metal7, located on the side of the tunnel-like furnace enclosure. However, it has been reported, for example in aforementioned U.S. Pat. No. 3,947,001, that direct exposure of the radiant heat from the burning fuel flame has to be avoided as it could cause overheating of the top layer of pellets. To overcome this possibility, it was proposed to install each burner in a small port extension located on the furnace tunnel, as illustrated in FIG. 1 of U.S. Pat. No. 3,947,001, thus shielding the pellets from the radiant heat from the burners.
As again illustrated in FIG. 1 of U.S. Pat. No. 3,947,001, Leighton proposed two different burner installations: on the right of this figure, the burner is installed in a vertical shaft, while on the left of the figure, the burner is horizontal. In both cases the pellets are shielded from the burner flame radiant heat.
In the case of the processing of pellets fired in a kiln, such as, for example in a Metso plant, it is not possible to prevent exposure of the pellets to the radiant heat from the flame of the kiln single burner installed on the kiln axis, although tumbling of the pellets in the rotating kiln may minimize overheating.
The combustion, in the burners, of natural gas, heavy oil and/or pulverized coal results, unfortunately, in the production of very important amounts of CO2. For example, a typical pellet plant producing annually 10 million metric tonnes of pellets emits about one million metric tonnes of CO2; therefore, the total world pellet production of 400 million metric tonnes of pellets corresponds to the production of about 40 million metric tonnes of CO2 per year.
Once the very hot combustion gases have gone through the bed of pellets being fired, they are used to preheat and dry the green oncoming pellets before being released to the atmosphere. The released gas temperature being about 350° C., the process energy efficiency would be of only about 80%.
Furthermore, the combustion gases, which are used to dry the ore pellets, are, in addition to CO2, composed of water vapor, which is not an optimal composition as a drying gas.
Because of the important release of green-house gases, of the low energy efficiency of the heating process and of the fact that a wet gas (that is a combustion gas containing water vapor in addition to carbon dioxide and nitrogen) is used in the drying zone of the furnace, there is a need for an improved technology for the induration of iron ore concentrate pellets.